Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751848AbdF1WOn (ORCPT ); Wed, 28 Jun 2017 18:14:43 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:53858 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751518AbdF1WOh (ORCPT ); Wed, 28 Jun 2017 18:14:37 -0400 Date: Thu, 29 Jun 2017 00:13:44 +0200 (CEST) From: Thomas Gleixner To: zhong jiang cc: Ingo Molnar , akpm@linux-foundation.org, mingo@redhat.com, minchan@kernel.org, mhocko@suse.com, hpa@zytor.com, x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative In-Reply-To: <595331FE.3090700@huawei.com> Message-ID: References: <1498045437-7675-1-git-send-email-zhongjiang@huawei.com> <20170621164036.4findvvz7jj4cvqo@gmail.com> <595331FE.3090700@huawei.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1882 Lines: 58 On Wed, 28 Jun 2017, zhong jiang wrote: > On 2017/6/22 0:40, Ingo Molnar wrote: > > * zhong jiang wrote: > > > >> when shift expoment is negative, left shift alway zero. therefore, we > >> modify the logic to avoid the warining. > >> > >> Signed-off-by: zhong jiang > >> --- > >> arch/x86/include/asm/futex.h | 8 ++++++-- > >> 1 file changed, 6 insertions(+), 2 deletions(-) > >> > >> diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h > >> index b4c1f54..2425fca 100644 > >> --- a/arch/x86/include/asm/futex.h > >> +++ b/arch/x86/include/asm/futex.h > >> @@ -49,8 +49,12 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr) > >> int cmparg = (encoded_op << 20) >> 20; > >> int oldval = 0, ret, tem; > >> > >> - if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) > >> - oparg = 1 << oparg; > >> + if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) { > >> + if (oparg >= 0) > >> + oparg = 1 << oparg; > >> + else > >> + oparg = 0; > >> + } > > Could we avoid all these complications by using an unsigned type? > > I think it is not feasible. a negative shift exponent is likely > existence and reasonable. What is reasonable about a negative shift value? > as the above case, oparg is a negative is common. That's simply wrong. If oparg is negative and the SHIFT bit is set then the result is undefined today and there is no way that this can be used at all. On x86: 1 << -1 = 0x80000000 1 << -2048 = 0x00000001 1 << -2047 = 0x00000002 Anything using a shift value < 0 or > 31 will get crap as a result. Rightfully so because it's just undefined. Yes I know that the insanity of user space is unlimited, but anything attempting this is so broken that we cannot break it further by making that shift arg unsigned and actually limit it to 0-31 Thanks, tglx